The structure of intracellular SV40 nucleoprotein complexes (SV40 chromatin) will be studied employing certain bacterial restrictions enzymes. Initial experiments have shown that SV40 chromatin becomes resistant to cleavage by restriction endonucleases after a single break is made in the SV40 DNA. Whether this phenomenon is a consequence of the SV40 chromatin having only certain specific sites available for cleavage or whether cleavage results in major structural rearrangements was not determined. In order to resolve this question and seek an explanation for the phenomenon the following types of studies will be carried out. 1) SV40 chromatin will be sequentially digested by more than one enzyme. These experiments will be done under different salt conditions which presumably can alter the configuration of the complexes. 2) The position of the single cleavage made by several enzymes will be mapped. 3) Native complexes and complexes with nicks in the SV40 DNA will be fixed then digested by non-restricted deoxyribonucleases. 4) Nicked and native SV40 chromatin will be examined in the electron microscope and by sedimentation velocity.